In order to satisfy the demand for transmission capacity within an available frequency band allocation, digital cellular systems divide a particular geographic area to be covered into a number of cell areas. A cell consists of a base station from which mobile units within the cell access the cellular system. It is the base station capacity that typically defines the optimal cell coverage area. The capacity of a base station is ideally as large as possible so that each cell can serve as an access point to the cellular system to as many mobile units as possible over a large area.
One method of achieving an increase in capacity is to replace a wide beamwidth antenna with an antenna array that provides a number of narrower beamwidth beams that cover the area of the original beam. Referring to FIG. 1, a conventional wireless communication cell 100 is shown comprising three adjacent sectors, alpha 102, beta 104 and gamma 106. Each cell comprises an antenna tower platform 120 located at the intersection of the three sectors. The antenna tower platform 120 has three sides forming an equallateral triangle. Each sector has three antennas, (only antennas in sector alpha 102 are shown) a first antenna 114, a second antenna 116, and a third antenna 112 mounted on a side of the antenna tower platform 120. The three antennas of each sector produce a corresponding set of three beams (only beams in sector alpha 102 are shown) including a first beam 108, a second beam 110 and a third beam 112. The three beams 108, 110, 112 are adjacent with some overlap. The three sectors alpha 102, beta 104 and gamma 106 are identical in structure with respect to antennas and beams. The signal for a particular user can then be sent and received only over the beam or beams that are useful for that user. If the pilot channel on each beam is unique (e.g. has a different PN (pseudo-random noise) offset) within each sector then the increase in capacity is limited due to interference between reused pilot channels in different cells.
An improvement is to use multiple narrow beams for the traffic channels and transmit the overhead channels (pilot, synch, and paging channels) over the whole sector so that the overhead channels are common to all the narrow beams used by the traffic channels in that sector. This leads to substantial gains in capacity. It is therefore desirable that the overhead channels be broadcast over the area covered by the original wide beam.
Broadcasting the overhead channels over an entire sector can be accomplished by using the original wide beam antenna or by transmitting the overhead channels synchronously using the same multiple narrow beams used to transmit and receive the traffic channels. However, a problem common to both of these arrangements is that both require the expense of extra hardware, complex calibration equipment and algorithms to match the phases of the overhead channels to the phases of the traffic channels.
Currently, multiple beams of one polarization are used to provide coverage for a single sector, with a second polarization used for diversity purposes. When a full sector transmission is required, as for the overhead channels, transmission of identical signals on all beams simultaneously can create spatial interference nulls at the beam crossover points, assuming that the equipment has not been carefully calibrated (so the relative phases are not controlled). An approach that is proposed in commonly assigned U.S. patent application Ser. No. 09/733,059, entitled “Antenna Systems With Common Overhead For CDMA Base Stations” and filed on Dec. 11, 2000 by McGowan et al, provides a method of phase cycling of the beams to ensure that a spatial null only persists for a short duration of time.
There is thus a desire to provide an antenna array that uses fixed narrow beams for transmitting and receiving the traffic channels on multiple beams and may broadcast the common pilot channel over all of the sector using the same antenna array. Furthermore, it would be advantageous to provide an antenna system that did not require complex calibration and adjustment to maintain performance over time and temperature.